1. Tritium labelled oestradiol-3 beta-D-glucuronide (E2-3G) was synthesised by sodium borohydride reduction of labelled oestrone-glucuronide (E1-G) and injected intravenously into anaesthetised rats. Bile and urine were collected to assess the routes and rate of excretion of E2-3G. Bile and urine samples were analysed by reverse phase HPLC to determine the metabolites of E2-3G. 2. When E2-3G was given at 11 and 22 mumol/kg, 83 and 85% respectively was excreted in bile within 3 hr and 1 and 3% in urine. 3. The major metabolite was E1-G which accounted for 89 and 92% respectively of the injected E2-3G which was recovered in bile. 4. It is concluded that bile is the major route of excretion of E2-3G in rats and that it is converted mainly to E1-G before excretion.